1. Field of the Invention
The invention relates to papermakers felts and more particularly relates to a wet press felt for use in the press section of a papermaking machine and the method of its fabrication.
2. Brief Description of the Prior Art
The conventional papermaking machine can be described as a highly sophisticated means of removing water from a dispersion of paper furnish. The machine includes three distinctly separate sections, beginning with the forming section where the furnish is deposited on a travelling forming wire and initially dewatered. The web of paper formed is conveyed into the wet press section for dewatering and then into the dryer section for final drying or removal of residual water by evaporation.
An important part of the process of papermaking is the efficiency of dewatering in the wet press section. The higher the efficiency of water removal in this section, the less will be the energy requirement in the dryer section.
In the wet press section of the papermaking machine, the formed web of paper is carried by one or more endless press felts through one or more presses which force the water out of the paper web and into or through the press felt. In the past, the press felts employed to carry the paper web through the press or presses have been absorbent woven and needled fabrics which are relatively resilient and compressible throughout their thickness. A wide variety of natural and synthetic fibers, yarns, woven and non-woven fabrics have been put together in a wide variety of combinations to fabricate wet-press felts. The objective is to arrive at a combination of felt components which will receive a maximum volume of water from the paper web as web and felt are compressed together in the nip of the press or presses, retain this water as the web and felt pass from the press nip (to minimize rewetting of the paper web) and then release the water before entering the press again. All of this must be accomplished by the felt fabric within the further requirements of structural integrity, runability, proper weight, resistance to filling with paper debris, resistance to compaction and like properties. As those skilled in the art fully appreciate, most of the fabrics employed to make wet-press felts are compromises, adequate in one or more of the requirements but excelling in one or more of other desired physical properties.
Current theory maintains that if the wet-press fabric were more dense, harder and more resistant to compression, there would be an enhancement of water removal. However, if the fabric had greater density and resistance to compaction than now provided in wet-press felts, it would seem that the fabric would have to have a lower void volume and less air permeability. Thus, another compromise would be necessary in order to take this approach to enhancing water removal.
Prior art attempts to obtain greater density in wet press felt fabrics have included installing them on papermaking machines and compacting them during a break-in period. The phenomenon of "break-in" of a papermaker's felt on a paper machine has been recognized for a long time. The so-called "break-in" period is usually defined as that time just after a new press felt has been installed on the paper machine (when its performance is less than optimum). The "break-in" period can last from several hours to a week and is usually accompanied by one or more of the following: (1) lower solid content in the paper after the nip, (2) harder drying, (3) operating problems such as blowing, picking and drop-offs, and (4) inability to run at top speed.
Although beneficial, felt compaction during "break-in" periods on the paper machine is expensive, troublesome and undesirable to the papermakers. Break-in time on the paper machine slows production and causes numerous quality and sheet handling problems. A new felt is more susceptible to filling and subsequent premature blinding during its initial faster rate of compaction, i.e., the break-in period. This is due to the fact that the large pores in a bulky new felt more readily occlude with paper stock, fines, fillers, etc., while they are being made smaller during compaction.
Precompaction has also always been a part of felt making, ever since all wool felts were run in a "kicker" and fulling mill to make them denser. More recently, it was found that all synthetic needled felts could be further densified by rope washing them under a squeeze roll or by applying enough heat and pressure to permanently deform and harden the felt in a nip on a dryer.
In all the cases mentioned above, work is merely being performed on the felts and results in a closer packing of the fibers and yarns to produce a denser and harder felt.
Methods other than precompaction have also been used by feltmakers to increase the felt's resistance to compaction on the paper machine e.g. dense and multi-ply base fabrics, low batt/base ratios and chemical treatments. All of these have helped to some degree.
Although precompaction methods employed by the feltmakers to date have been somewhat effective in allowing faster startups on the paper machine, they have been limited in the degree of compaction. This is mainly due to the fact that during compaction, the felt density and hardness increases with a resulting decrease in void volume and permeability. In other words, the felt becomes filled up with itself. If this is carried too far, the felt will no longer function as a porous capillary structure and loses its water handling properties. It would be so hard and stiff, the papermaker probably would not be able to install it on his machine.
With the current trend toward higher press loadings and faster speeds on paper machines, the development of a compaction resistant press felt has become imperative. A new method of achieving this has been developed whereby a felt can be precompacted while controlling its void volume, by the method of the present invention. This new method incorporates the use of a relatively non-compressive base fabric and a web blend containing, in part, a solid fugitive material. After needling, the felt is subjected to heat and pressure, thereby increasing both its density and hardness to an appreciably higher degree than current precompaction methods allow. The fugitive material is then removed from the super compacted felt in order to regain the lost void volume, permeability and water handling properties.
The fabrics and method of the present invention obviate the expected prior art problems and permit the manufacture and use of wet-press felts having greater density and higher degrees of incompressibility, without significant reduction of void volumes or permeability. By the method of the invention, the compressive modulus and elasticity of the felt can be permanently modified without loss of void volume or air permeability. The felts of the invention improve dewatering efficiency of the papermaking machine.
The prior art literature is replete with descriptions of prior art wet press felts, belts made therefrom and their use in papermaking machines. Representative of such description are those found in U.S. Pat. Nos. 2,883,734; 2,907,093; 3,097,413; 3,401,467; and 3,458,911.